Terminals (User Equipments/Mobile Stations) are often equipped with capabilities for access via 2G/3G and LTE (Long Term Evolution). Other equipments, such as a laptop, may have capabilities for access via HSPA (High Speed Packet Access)/LTE (Long Term Evolution), WLAN (Wireless LAN) and Ethernet. Typically the terminal or laptop uses different accesses one at a time. And more important, a given service or a given IP session is only using one access at a time.
In a Gn-SGSN (Serving Gateway Support Node), which is an SGSN not supporting S4 interface but only Gn/Gp interface, it is possible for the mobile station/user equipment (MS/UE) to activate PDP (Packet Data Protocol) contexts towards a GGSN and towards a PGW simultaneously. The PGW has Gn/Gp interface and thus acts as a GGSN over that interface. The 3GPP standard allows for this simultaneously existence of PDP contexts towards GGSN and PGW in a Gn-SGSN. However, this is prevented in a S4-SGSN.
In EPC (Evolved Packet Core) the MME (Mobility Management Entity) has no interface towards the GGSNs. Since the MME has no interface towards the GGSN, there is a problem when the MS/UE moves to EPC. The PDP contexts in the GGSN can't be handled by MME and there exist no possible way to remove them. The only option to solve this is to reject the whole TAU procedure when moving to EPC if the MS/UE simultaneously has activated PDPs towards GGSN and PGW.